[pgAdmin4][patch] fix a typo in import_export_servers.rst

Hi, all

It seems like there's a following typo in import_export_servers.rst
-line option. There can be multiple configuations of pgAdmin on the same system.
+line option. There can be multiple configurations of pgAdmin on the same system.

Attaching a tiny patch to fix it. 
Please review.

Regards,
zhang jie

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